X-ray crystal structures of the P-clusters of Av MoFeP variants. Anomalous electron density difference maps determined using X-ray diffraction data collected near the Fe K-edge are shown in black mesh. (a) Av βS188A P-cluster in DT-reduced (left) and IDS-oxidized (middle) states (7.13 keV, Fe f” = 3.93 and 12.0 keV, Fe f’ = 1.64). Maps are contoured at 8.0 and 3.5 σ, respectively. Fe1 and Fe5 are both redox-labile (right). (b) Av βF99Y/βS188A P-cluster in DT-reduced (left) and IDS-oxidized (middle) states (7.13 keV, Fe f” = 3.93 and 8.0 keV, Fe f” = 3.18). Maps are contoured at 8.0 and 6.0 σ, respectively. Fe1, Fe5, and Fe8 are partially occupied (right). (c) Av βF99Y P-cluster in DT-reduced (left) and IDS-oxidized (middle) states (7.13 keV, Fe f’ = 3.93 and 13.0 keV, Fe f’ = 1.43). Maps are contoured at 3.5 σ. This mutant contains no redox-labile iron centers (right).